Weird cloth physics on bends
First:
 I would apply modifiers before you run simulations, so you could try applying the decimate modifier and re-running the simulation.

Second:
You could subdivide the cloth some more, in edit mode. I'm not sure if you did this, but subdividing the cloth using the SubSurf modifier doesn't increase the resolution of the cloth in the calculations of the simulation.

Third:
Increase the "padding" of the object. By "padding", I mean the distance from the object that the cloth will colide with. I don't know what yours currently are, but hopefully tweaking this will help. The values 'Inner' and 'Outer' are what controlls this.